1. Field of the Invention
The present invention relates to a process for bleaching a lignocellulose pulp. More preferably, the present invention relates to a process for bleaching a lignocellulose pulp, which enables a consumption of auxiliary chemicals for bleaching to be reduced to a great extent.
2. Description of the Related Art
It is known that in an alkali-oxygen bleaching (an oxygen delignification) process, a pulp is bleached in a reaction vessel by heat-treating the pulp with an alkali and oxygen placed in the vessel under pressure to produce radicals of lignin and resin in the pulp and to oxidize-decompose the radicals of lignin and resin. In the alkali-oxygen bleaching process, currently a moderate consistency oxygen-bleaching process (pulp consistency=8 to 15% by weight) is mainly used, in view of the relationship between the cost of bleaching apparatus necessary in this process and the quality of the resultant pulp. This process is advantageous in that a COD load on the environment is low and a non-chlorine bleaching agent can be used in a reduced amount in rear stage or stages in a multiple stage bleaching step, and thus is utilized in many factories in the world. However, the alkali-oxygen bleaching (an oxygen delignification) process is disadvantageous in that when the lignin in the pulp is removed in an amount of about 50% by weight based on the total content of lignin, the pulp cellulose is significantly damaged, the yield of the pulp is reduced and the viscosity of the pulp is decreased. This disadvantage can be restricted to a certain extent by using a magnesium salt as an agent for restricting the decomposition of cellulose. However, this restriction of the cellulose decomposition is not sufficient in practice. Thus, to keep the viscosity of cellulose at a practically permissible level or more, the lignin must be retained in a certain content in the alkali-oxygen-bleached oxygen-delignified pulp, and thus the bleaching efficiency of the conventional alkali-oxygen bleaching process is not always satisfactory. Accordingly, an enhancement in the bleaching efficiency of the alkali-oxygen bleaching process greatly contributes to reducing the load on the environment and to decreasing the bleaching cost due to the bleaching chemicals.
There have been many attempts to improve the alkali-oxygen bleaching process. For example, Japanese Unexamined Patent Publication No. 4-272,289 discloses an improved alkali-oxygen bleaching (oxygen-delignification) process in which two alkali-oxygen bleaching (oxygen-delignification) apparatuses are arranged in series and a washing means is inserted between the two bleaching apparatus. Also, U.S. Pat. No. 4,946,556 (Japanese Unexamined Patent Publication No. 3-14,686) discloses an alkali-oxygen bleaching (oxygen-delignification) process using a plurality of alkali-oxygen bleaching (oxygen-delignification) apparatuses arranged in series and a plurality of washing means respectively attached to each of the bleaching apparatuses. In these processes, merely the waste liquid delivered from each alkali-oxygen bleaching apparatus is washed by a countercurrent washing liquid and then is recovered into a pulp production step, and thus, the efficiency in delignification by the alkali-oxygen bleaching (oxygen-delignification) procedures and the whiteness of the bleached pulp are not satisfactorily enhanced.
Recently, various attempts have been made to reduce the load on the environment and to decrease the amounts of the bleaching and auxiliary chemicals employed in the rear stage or stages in the multiple stage bleaching procedure. In one attempt, a bleaching procedure using an enzyme, for example, xylase has been developed. For example, a bleaching method in which a pulp is treated with xylanase before the multi-stage bleaching procedure, is disclosed, for example, in Japanese Unexamined Patent Publication No. 2-264,087 (corresponding to U.S. Pat. No. 5,179,021), No. 2-293,486 (corresponding to European Patent No. 395,792) and No. 4-507,268 (corresponding to WO 91/02,840). Also, a bleaching method in which a pulp is treated with a lignin-decomposing enzyme before bleaching procedure, is disclosed in Japanese Unexamined Patent Publication No. 2-500,990 (corresponding to WO 88/03,190, No. 3-130,485 (corresponding to European Patent No. 408,803, and No. 4-316,689 (corresponding to U.S. Pat. No. 5,618,386).
The treatment of the pulp with the enzyme before the bleaching procedure is advantageous in that the enzyme treatment conditions are relatively moderate and thus the reduction in the mechanical strength and the yield of the bleached pulp is slight, but is disadvantageous in that the reaction rate is low, and thus a long time is necessary to complete the enzyme reaction, and the reduction in Kappa value of the bleached pulp is very small.
Recently, the treatment of the pulp with xylanase particularly has drawn the attention of the paper industry. In the xylanase treatment, the enzyme must be brought into close contact with the pulp fibers to generate the reaction of the enzyme. However, since xylan and lignin contained in the pulp fibers are polymeric and are unevenly distributed in three dimensions in the pulp fibers, and the xylanase per se is polymeric, it is difficult to bring the xylanase into close contact with the xylan or lignin distributed in the pulp fibers. Thus, a new method of carrying out the enzyme reaction with a high efficiency must be developed.
The utilization of the enzyme including xylanase for the paper and pulp industry is disclosed in detail in Pratima Bajpai, xe2x80x9cEnzyme in Pulp and Paper Processingxe2x80x9d, published in 1998 by Miller Freeman Inc. Also, L. Viikari et al., xe2x80x9cBiotechnol. Pulp Paper Ind. (Stockholm), pages 67 to 69, 1986, discloses a treatment of pulp with xylanase, and reports that the bleaching efficiency of pulp was improved by the xylanase treatment. Further, F. Mora et al., xe2x80x9cJournal of Wood chemistry and Technologyxe2x80x9d, (6) 2, pages 147 to 165, 1986, reported that the treatment of pulp with xylanase after the pulp was bleached with oxygen contributed to enhance the mechanical strength of the bleached pulp. These reports are, however, quite silent as to the utilization and recovery of a waste liquid delivered from the enzyme treatment system.
In the bleaching procedures wherein a hemicellulase, for example, xylanase, is used and the bleached pulp is washed by a countercurrent washing method, the resultant bleaching reaction product mixture contains organic substance produced by the reaction of the enzyme with the pulp material and containing saccharide as a main component, and the saccharide-containing organic substance causes the countercurrent washing procedure to be difficult. Particularly, where the hemicellulase treatment is applied to the pulp material after the alkali-oxygen bleaching procedure, the organic substance containing saccharide produced by the hemicellulase treatment is returned into the alkali-oxygen bleaching (oxygen delignification) procedure through the countercurrent washing procedure, since a waste liquid delivered from the washing procedure is returned into the alkali-oxygen bleaching procedure.
It is well known that in the alkali-oxygen bleaching procedure, oxygen radical generated under the alkalin condition reacts with organic substances other than lignin in the pulp and having reduction-functional groups. The reaction mixture delivered from the hemicellulase treatment contains a large amount of fragments of decomposed lignin, and polysaccharides, oligosaccharides, monosaccharides, resin acid and derivatives thereof, and is washed by the countercurrent washing procedure, and the waste liquid delivered from the washing procedures and containing the above-mentioned organic substances is returned into the alkali-oxygen bleaching procedure. In this case, the saccharide molecules contained in the returned waste liquid have aldehyde groups which exhibit a reduction property. Thus in the alkali-oxygen bleaching reaction system, the returned saccharides react with oxygen so that the oxygen supplied into the bleaching system is wastefully consumed. Also, the saccharides reacted with oxygen are oxidized and converted to organic acids. The resultant organic acid molecules have carboxyl groups which cause the pH value of the bleaching system to be shifted to acid side, and thus the bleaching activity of the alkali-oxygen bleaching system is deteriorated. In this condition, to maintain the pH value of the bleaching system within a high alkalin range, and the delignification efficiency of the bleaching system at a high level, the alkali and oxygen must be respectively fed in increased amounts into the bleaching system. For this purpose, an attempt has been made to increase the amount of the white oxidation liquid fed into the bleaching system and to supplement the alkali consumed by the reaction with the saccharides. However, this attempt is disadvantageous in that the cost of the pulp production is increased, the delignification efficiency is unsatisfactory and the Kappa value of the resultant pulp is not satisfactorily low.
Also, according to Japanese Unexamined Patent Publication No. 63-112,979, in a method of recovering xylooligosaccharide from a filtrate of a reaction mixture prepared by treating hardwood xylan with xylanase derived from Trichoderma, the filtrate is decolored by activated carbon, the activated carbon is removed from the filtrate by using a filter press, the saccharide absorbed in the activated carbon is recovered by using a 15% ethanol, the recovered saccharide is treated with an ion-exchange resins (trademark: AMBERLITE IR-120B and AMBERLITE IR-410, to remove salts, and then is concentrated by a reverse osmosis membrane to obtain xylooligosaccharide containing xylobiose in a high content.
These publications are, however, quite silent as to the recovery and refining of xylooligosaccharides from a filtrate prepared from a reaction mixture in which a chemical pulp is treated with hemicellulase.
It is known that the xylanase treatment applied to the kraft pulp enables the necessary amount of bleaching chemicals for the bleaching process for the pulp with the bleaching chemical to be reduced. In the xylanase treatment, since the xylan contained in the pulp is hydrolyzed with xylanase, the resultant waste water discharged from the bleaching system contains xylose and xylooligosaccharide separated from the pulp in large amount. In paper industry, to reduce the amount of process water used, an amount of water used in a step of the bleaching procedure is returned to and utilized in another step before the above-mentioned step. Therefore, the water used in a step before the enzyme treatment step contains xylan-decomposition products, for example, xylose and xylooligosaccharide, isolated by xylanase.
The above-mentioned xylose and xylooligosaccharide have reducing terminal groups, for example, aldehyde groups, the reducing terminal groups are oxidized in the oxidation-bleaching procedure, for example, an oxygen-bleaching procedure and the xylose and xylooligosaccharide are converted to carboxylic acids and further to oxidized furan derivatives and then to colored furan condensation products, to consume the bleaching chemicals. Thus, in this case, the bleaching agents consumed due to the presence of the saccharides must be supplemented. Also, in the oxygen bleaching procedure under a high alkaline condition, the aldehyde groups are oxidized and the resultant carboxylic acid causes the pH value of the bleaching system to be reduced. Thus the pH values of the bleaching system must be controlled to a desired level by increasing the amount of alkali to be added to the bleaching system to compensate the reduction in pH.
In an attempted method in which xylose and xylooligosaccharide produced by the xylanase treatment is not returned to a preceeding bleaching step, the reducing saccharides are removed from the waste water discharged from the enzyme treatment system, and the resultant saccharide-free waste water is returned to a preceeding bleaching step. However, the waste water from the pulp production is generated in a large amount, and thus the removal of the saccharide by a conventional method, for example, the reverse osmosis membrane method, causes a very large scale of apparatus to be provided. Therefore, the above-mentioned removal of saccharide has not yet been carried out at a low cost.
An object of the present invention is to provide a process for bleaching a lignocellulose pulp with a high efficiency, while utilizing a waste water delivered from an enzyme treatment step as a liquid medium of an alkali-oxygen bleaching step.
Another object of the present invention is to provide a process for bleaching a lignocellulose pulp, by bleaching a pulp by an alkali-oxygen bleaching procedure and treating the pulp with an enzyme, which process enables a waste water delivered from, in a countercurrent to, the enzyme treatment, for example, hemicellulose treatment to be returned to a preceeding alkali-oxygen bleaching step, without deteriorating the bleaching effect of the alkali-oxygen bleaching procedure.
Still another object of the present invention is to provide a process for bleaching a lignocellulose pulp, while recovering xylooligosaccharide contained in a waste liquid delivered from an enzyme treatment step with a high efficiency and in a low cost, and while preventing a reduction in bleaching effect due to the presence of the xylooligosaccharide, in an alkali-oxygen bleaching system.
The above-mentioned objects can be attained by the process of the present invention for bleaching a lignocellulose pulp.
The bleaching process of the present invention for a lignocellulose comprises the steps of:
(1) bleaching a pulp in an aqueous alkali solution with oxygen; and
(2) enzyme-treating the pulp with hemicellulase to an extent such that a liquid fraction containing succharides in an total amount of 2 to 6 mg/ml is produced in the resultant reaction mixture;
(3) filtering the resultant reaction mixture delivered from the enzyme-treating step (2) to recover the enzyme-treated pulp separated from the liquid fraction of the reaction mixture;
(4) subjecting the liquid fraction delivered from the filtration step (3) to a permeation treatment through a separation membrane to separate a permeated fraction from a non-permeated fraction; and
(5) feeding the resultant permeated fraction delivered from the permeation treatment step (4) and containing succharides in a total content of 0.3 to 1.2 mg/ml, to the alkali-oxygen bleaching step (1) to use it as a liquid medium of the alkali-oxygen bleaching step (1).
In the bleaching process of the present invention, preferably the enzyme treatment step (2) is carried out after the alkali-oxygen bleaching step (1).
In the bleaching process of the present invention, the enzyme treatment is preferably carried out by using, as a hemicellulase, xylanase.
In the bleaching process of the present invention, the permeation treatment using the separation membrane is preferably carried out by using a membrane for reverse osmosis or for nanofiltration.
In the bleaching process of the present invention, the liquid fraction of the filtration step (3) is mixed with a flocculant selected from the group consisting of inorganic flocculants and polymeric flocculants, the resultant flocculate is removed from the liquid fraction, the flocculate-free filtrate is subjected to the permeation treatment step (4) using the separation membrane.